Antennas have been used on automobiles for many years. Originally, antennas were installed on automobiles to allow for reception of signals for the car radio. A whip antenna protruding from one of the vehicle fenders for radio reception was standard on most automobiles. Later, antennas that were either embedded within or affixed to the inside of the windshield of the automobile were developed. These in-glass or on-glass antennas ran around the perimeter of the windshield and were less visible than the whip antennas and less susceptible to damage from external elements such as weather or vandalism.
Today, complicated on-board communication systems are used in the automotive industry. Vehicle manufacturers offer systems with features such as built in telephone communication and global positioning satellite (GPS) systems. With the introduction of these complex systems, there was a corresponding increase in the complexity of the antennas required. These systems require antennas that can both receive and transmit signals on several frequency bands. The Personal Communication Service (PCS) band and the Advance Mobile Phone Service (AMPS) band are the most common frequency bands used in cellular telephone communication, with the PCS band used primarily for digital transmissions and the AMPS band used primarily for analog transmissions. Global positioning satellite systems operate within a third distinct frequency band known as the GPS band.
Several types of antennas have been used in conjunction with these kinds of communication systems. Single pole, dipole and slot antennas are examples of well known types of antennas used. The predominant mode of reception for these systems is vertical polarization. Single pole and dipole antennas provide polarization in the same direction as the orientation of the antenna, while slot antennas provide polarization perpendicular to the orientation of the antenna. For example, a standard single pole or dipole whip antenna would need to be vertically oriented to achieve the desired vertical polarization. A slot antenna would need to be horizontally oriented to provide the desired vertical polarization. Vertically oriented whip antennas have been used on the rooftop, fenders, and rear windshield of vehicles for mobile telephone reception for several years.
While the primary mode of polarization of PCS and AMPS signals is vertical in nature, many providers also offer diversity polarization. Diversity polarization means the signal can be switched between vertical, horizontal, and a form of slant polarization (slant polarization is on an angle between the two) to provide the best coverage in difficult coverage areas. Diversity polarization allows the system to account for the change in polarization resulting from the signal reflecting off of structures and the landscape.
External vertical whip antennas have several disadvantages. First, they are not aesthetically desirable. Also, they are easily susceptible to damage from external forces such as weather, vandalism, and automatic car washes. There exists a desire among vehicle designers to remove the external whip antennas and replace them with on-glass antennas in a manner similar to what had been done previously for radio reception.
On-glass antennas for the complex communication systems used today created a new set of problems. Dipole antennas are unsuitable for on-glass applications for several reasons. First, in order to achieve the desired vertical polarization, the antenna would need to be vertically oriented on the windshield glass. Vertical orientation of this type of antenna on the front windshield is a concern because it causes an obstruction of the vehicle operator's view. In addition, dipole antennas radiate omni-directionally in the plane perpendicular to the antenna axis, including backwards into the passenger compartment. Thus, the vehicle occupants are subject to the signal energy radiated from the antenna. Recently, there has been widespread concern about the possible negative effects of such radiation on humans. Adding some type of reflector shield to prevent RF signals from radiating backward into the passenger compartment is not practicable because of the size of the shield necessary and the obstruction to the operator's view that would result from adding such a shield. Finally, vertically oriented dipole antennas are not receptive to the diversity polarization signals.
Patch antennas with reflectors have been used because of their small size and directional nature; however, while it is desirable to avoid radiation traveling into the passenger compartment, omni-directional radiation outside of the vehicle is preferred for optimum antenna performance. For example, patch antennas by nature have a narrow beam width and as a result do not provide the desired performance for vehicular applications.